Soft handoffs are one of the greatest advantages of Code Division Multiple Access (CDMA) technology over other multiple access technologies being used for wireless communications systems because of the smooth and seamless transition provided to mobile-telephones traversing from one cell to another cell. Generally, a handoff involves the handling of a call by a progression of base stations to keep the call active as the mobile-telephone traverses from one cell to another cell. A soft handoff is a state in which a mobile-telephone maintains simultaneous radio links with multiple base stations. Referring to FIG. 1, there is shown a schematic of a wireless communication system 10 processing a call in soft handoff with two base stations. As shown in FIG. 1, transmissions to the mobile-telephone 11, i.e., forward link, are carried simultaneously by the base stations 12, 14 participating in the soft handoff. Similarly, transmissions from the mobile-telephone 11, i.e., reverse link, are processed (demodulated) by the base stations 12, 14 before they are appropriately combined at a common frame handler 15 and passed on to the Public Switched Telephone Network (PSTN) 17 via a mobile switching center (MSC) 16. This enables the mobile-telephone 11 to move from the coverage area of one base station to that of another without a user-detectable disruption of communication--that is, the transition is smooth and seamless between cells.
In IS-95 based CDMA wireless communication systems, a soft handoff is initiated by a mobile-telephone (that is engaged in a call) when the mobile-telephone detects a pilot signal from a neighbor base station with a signal strength exceeding a threshold T.sub.-- ADD. Upon detecting such a pilot signal, the mobile-telephone then transmits a pilot strength measurement message indicating the detection of a candidate base station, which is the neighbor base station with the pilot signal strength exceeding the threshold T.sub.-- ADD. Specifically, the pilot strength measurement message is transmitted "in-band" on a reverse traffic channel (i.e., traffic channel in the reverse direction) to a set of base stations (referred to herein as active set base stations) comprising of a primary base station and zero or more secondary base stations--that is, the pilot strength measurement message is transmitted on a reverse traffic channel which is being listened to (i.e., demodulated) by all the active set base stations.
All of the active set base stations receiving the pilot strength measurement message will demodulate the message and send the message to the common frame handler 15 where the (demodulated) pilot strength measurement messages are combined to obtain a single message. Subsequently, the (combined) pilot strength measurement message is passed to a call processor (CP) 18, which is an entity for processing signaling messages such as the pilot strength measurement message. Note that the frame handler and the call processor are typically located at a primary base station or MSC, and are well-known in the art. Upon receiving the pilot strength measurement message, the CP proceeds to set up network connections necessary for performing the soft handoff. Specifically, the setting up of network connections involves messaging between the CP, the candidate base station, and terrestrial network infrastructure of the wireless communication system, and establishing a link between the candidate base station and the frame handler. Typically, the CP sends a message to the candidate base station requesting a new traffic channel in the forward direction (i.e., forward traffic channel) be assigned to the mobile-telephone to enable communication with the candidate base station, and informing the candidate base station of the traffic channel being used by the mobile-telephone in the reverse direction (so the candidate base station knows what reverse traffic channel to which it should listen). At the same time, the CP interacts with the terrestrial network infrastructure to set up a link between the candidate base station and the frame handler (to enable proper routing of messages between the candidate base station and the frame handler).
Once these network connections are established and a forward traffic channel is assigned by the candidate base station (for communicating with the mobile-telephone), the CP sends to the active set base stations copies of a handoff direction message, which is a message instructing the mobile-telephone to add the candidate base station to the active set and informing the mobile-telephone what forward traffic channel has been assigned by the candidate base station for communicating with the mobile-telephone. Additionally, upon completion of the network connections setup, the candidate base station will begin to carry the call over the newly assigned forward traffic channel--that is, the candidate base station will carry copies of the forward link transmissions on the newly assigned forward traffic channel, and demodulate the mobile-telephone's reverse link transmissions (and pass them to the frame handler).
Once the candidate base station is added to the active set (by the mobile-telephone), the candidate base station becomes a secondary base station--that is, the mobile-telephone will now listen to the candidate base station in addition to all of the other base stations which were already in the mobile-telephone's active set. Assuming that the primary base station and the former candidate base (now a secondary base station) are the only base stations in the mobile-telephone's active set, the mobile-telephone will communicate with both of these base stations until the pilot signal strength of one of the base stations, as measured at the mobile-telephone, falls below a threshold T.sub.-- DROP for a period TT.sub.-- DROP. Upon the occurrence of such an event, the mobile-telephone sends an "in-band" pilot strength measurement message to the active set base stations indicating the detection of a weak base station, i.e., which is the active set with the pilot signal strength below T.sub.-- DROP for the period TT.sub.-- DROP. This pilot strength measurement message is subsequently used by the wireless communication system to have the weak base station "drop" out of the call and be removed from the active set--that is, the weak base station and the mobile-telephone will cease to communicate with each other.
Under rapidly fluctuating fading conditions, such as those due to building blockage, sharp terrain change, mobile speed, etc., field testing has revealed that the success rate of CDMA soft handoffs can decrease significantly. This significant decrease in success rate is due, in major part, to delays associated with messaging during network connections setup. Specifically, from the time the mobile-telephone transmits the pilot strength measurement signal (to the active set base stations) to the time the active set base stations transmit the handoff direction message (to the mobile-telephone), the mobile-telephone may move to a new position where the signal-to-noise ratio on the forward traffic channels between the active base stations and the mobile-telephone is significantly decreased, thereby inhibiting the mobile-telephone's ability to receive the handoff direction message--that is, the signal-to-noise ratio of the handoff direction message is significantly decreased. The greater the network connections setup time, the greater the likelihood of the mobile-telephone traveling to a position where the decreasing signal-to-noise ratio of the handoff direction message is significantly decreased. Under rapidly fluctuating fading conditions, this likelihood is further increased.
Significant decreases in the signal-to-noise ratio (of the handoff direction message) results generally from the movement of the mobile-telephone, during network connections setup, to a position where a new obstacle, e.g., a building, becomes situated along the path of the handoff direction message (from the transmitting active set base station to the mobile-telephone), or a position where signals transmitted by the candidate base station can be received at a significantly higher level resulting in increased overall interference level to the handoff direction message, e.g., closer to the candidate base station and farther from the active set base stations.
One way to alleviate this problem is to reduce the time necessary to complete the network connections setup. This reduces the distance a mobile-telephone can travel during the network connections setup, thereby decreasing the likelihood of the mobile-telephone traveling to a position with poorer signal-to-noise ratio (of the handoff direction message). However, such a solution is difficult to achieve because of hardware and communication resource limitations in the network infrastructure of existing wireless communication systems. Accordingly, there exists a need for increasing the success rate of soft handoffs using the network infrastructure of existing wireless communication systems, particularly under rapidly fluctuating fading conditions.